IEEE wireless access in vehicular environments (WAVE), which is a vehicle to vehicle (V2V) communication standard, uses a basic safety message (BSM) as the most basic transmission method to recognize surrounding vehicles. For safe driving and fast networking, the BSM may be maximally transmitted within the allowable transmission environment.
The greater the number of vehicles on the road, the more BSMs are generated. Therefore, the possibility that a busy situation occurs in a channel used in the WAVE is increased.
When a busy situation occurs, a method of reducing the strength of message transmission may be considered. However, the method of temporarily reducing transmission strength has potential drawbacks. To this point, an example of this method is illustrated in FIG. 1.
FIG. 1 is a conceptual diagram illustrating the hidden terminal problem (HTP) maintained when the transmission strength is reduced in response to the busy situation, as described above.
FIG. 1 illustrates vehicle A, vehicle B, and vehicle C, as well as a communication range corresponding to each of the vehicles. In particular, vehicle A may communicate with vehicle B, and vehicle B may communicate with vehicle C. However, vehicles A and C are disposed outside the communication range therebetween. Since carrier sensing and the like are performed based on vehicle A, vehicle A determines that vehicle B does not communicate with other vehicles (for example, vehicle C) to transmit its own message. This may also be applied to vehicle C.
Even though the transmission strength can be reduced in response to the busy situation, vehicles which have the HTP relation still exist. Therefore, the reduction in receiving rate of a successful message (for example, BSM) is inevitable.
Accordingly, in V2V communication or vehicle to infra/vehicle/nomadic communication (V2X), when the busy situation occurs, a serious problem in recognizing surrounding vehicles may occur. Due to the small size of the contention window, as defined in the IEEE 802.11p revised bill, the opportunity to avoid a media access control (MAC) message collision is limited. In particular, the contention increases at the initial stage of message transmission.